Implantable blood pumps may be used to provide assistance to patients with late stage heart disease. Blood pumps operate by receiving blood from a patient's vascular system and impelling the blood back into the patient's vascular system. By adding momentum and pressure to the patient's blood flow, blood pumps may augment or replace the pumping action of the heart. For example, a blood pump may be configured as ventricular assist device or “VAD.”
A VAD is a device which is used to assist the heart of a mammalian subject such as a human patient. A typical VAD includes a pump which is implanted in the body of the subject. The pump typically has an inlet connected to a source of blood to be circulated, and an outlet connected to an artery. Most typically, the inlet of the pump is connected to the interior of the left ventricle and the outlet of the pump is connected to the aorta, so that the pump operates in parallel with the left ventricle to impel blood into the aorta. The pump may be a miniature rotary impeller pump having an impeller disposed in a pump housing and driven in rotation by a small electric motor which may be closely integrated with the pump. The motor in turn typically is powered by an implantable power source such as a storage battery with an arrangement for charging the battery from an external power source. The VAD typically includes a control system which controls operation of the power source so as to drive the impeller at a set rotational speed and thus provide constant pumping action. VADs can be used to assist the heart of subjects suffering from conditions which impair the pumping ability of the heart. Such assistance can be provided permanently, or while the subject awaits a suitable heart transplant. In other cases, the assistance provided by the VAD allows the heart to heal.
Starting up operating of a rotary pump, such as the rotary impeller pump of a VAD, may be difficult, particularly when the rotor is at rest up against the housing of the pump. Where the rotary pump is off and/or temporarily ceases operation, it is desirable to be able to start or resume operation of the pump as quickly and as efficiently as possible so as to provide assistance to the subject's heart. However, ramping the speed of the pump's rotor too quickly can be counterproductive. For instance, if the pump speed is increased too quickly, it can result in phase commutation errors. Further, it can create suction within the subject's ventricle, wherein the ventricle is collapsed and essentially devoid of blood. This condition may in turn cause the flow rate of blood through the pump to decline rapidly and to the extent that the flow rate is insufficient, the pump cannot provide sufficient circulatory assistance to the subject. Thus, these conditions are undesirable.
Additionally, ramping the speed of a pump requires a supply of power to be provided to the pump. However, supplying too much power too quickly may cause the control electronics to overheat. Aside from any safety hazards associated with overheating of control circuitry, in many cases, such overheating can also trigger a cutoff, which in turn hinders the controller's ability to start up or resume operation of the pump. Thus, this scenario is also undesirable.